This invention relates generally microelectromechanical systems (MEMS). More particularly, forming conductive landing pads on MEMS structures.
Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers (known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate and move into the path of the light from the fiber. Mirrors that move into the light path generally use torsion flexures to translate mirror position vertically while changing its angular position from a horizontal to a vertical orientation. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these. The design, fabrication, and operation of magnetically actuated micromirrors with electrostatic clamping in dual positions for fiber-optic switching applications are described, e.g., by B. Behin, K. Lau, R. Muller in xe2x80x9cMagnetically Actuated Micromirrors for Fiber-Optic Switching,xe2x80x9d Solid-State and Actuator Workshop, Hilton Head Island, S.C., Jun. 8-11, 1998 (p. 273-276).
When the mirror is in the horizontal position, it rests against a substrate that forms a base. Often, the mirror is subject to electromechanical forces, sometimes referred to as xe2x80x9cstictionxe2x80x9d that cause the mirror to stick to the substrate and prevent the mirror from moving. The same stiction forces can also prevent the mirror from being properly released from the substrate during manufacture. To overcome stiction problems, landing pads (also called dimples or bumps have been used extensively in MEMS devices to minimize or otherwise control the contact area between the device and the underlying substrate. In the prior art, such landing pads are formed prior to deposition of a device layer either by etching pits in an underlying sacrificial layer or by depositing pads of another material prior to the deposition of the layer forming the device.
Recently, silicon on insulator (SOI) techniques have been developed for fabricating MEMS devices. In SOI, an oxide layer is grown or deposited on a silicon wafer. A second silicon wafer is then bonded to the oxide layer, e.g. by plasma bonding. After bonding, the second silicon wafer is cleaved such that a thin layer of silicon is left attached to the oxide layer to form an SOI substrate. However, when that thin silicon layer is a MEMS device layer it is generally not possible to process the underside of the device layer prior to bonding the device layer to the oxide layer. Any processing of the device layer must therefore be done after it is attached to the underlying substrate. However since the underside of the device layer is firmly attached to the oxide layer it is not normally possible to deposit material on or etch material from the underside of the device layer. Currently, no technology exists for forming pads on the underside of the device layer of a MEMS device fabricated using SOI.
There is a need, therefore, for an SOI MEMS device having landing pads on an underside of the device layer and a method of fabricating same.
The problem of stiction with respect to an exemplary MEMs mirror device 800 is shown in FIG. 8. The device 800 includes a mirror 811 formed from the device layer 812 of a substrate 810. The mirror 811 may be movably attached to the device layer by a flexure 814, actuated by an. off-chip electromagnet, and individually addressed by electrostatic clamping either to a surface of the substrate 810 or to a vertical sidewall 804 of a top mounted chip 806. Magnetic actuation may move the mirror 811 between a rest position parallel to the substrate 810 and a position nearly parallel to the vertical sidewall 804 of the top-mounted chip 806, while the application of electrostatic field may clamp the mirror 811 in the horizontal or vertical position. The electrostatic field used to hold the mirror 811 in a position regardless of whether the magnetic field is on or off can increase the level of stiction between the mirror 811 and each landing surface.
When clamped to either the substrate 810 or the vertical sidewall surface 804, the mirror rests on a set of landing pads or dimples 822, 824, which may protrude below or above the mirror surface, respectively. These landing pads 822, 824 minimize the physical area of contact between the mirror 811 and the clamping surface, thus reducing stiction effects. However, since the mirror 811 and clamping surface (either the side wall 804 or the substrate 802) are at different potentials, the landing pads 822, 824 are made of an insulating material in order to prevent an electrical short between the mirror 811 and the clamping surface. While the insulating landing pad material does, indeed, prevent an electrical short, its inherent properties can lead to other problems. Firstly, most insulating materials have the capacity to trap electrical charge and can, in some cases, maintain that charge for long periods of timexe2x80x94sometimes indefinitely. As a result, the potential of the landing pads 822, 824 can drift to an arbitrary value, resulting in either parasitic clamping potential between the mirror 811 and the clamping surface, even when both are externally driven to the same voltage, or a reduced clamping force by shielding the mirror potential. Second, since the insulating landing pads 822, 824 will typically be at a potential close to the mirror potential when not in contact with the clamping surface, a rapid discharge can occur when the landing pads 822, 824 first come into the contact with the clamping surface that is a kept at a potential different than the mirror 811. This rapid discharge may be exhibited as arcing or short pulses of high current. Such surges can lead to physical damage to the landing pads 822, 824 or the clamping surface, or may produce micro-welding, where the landing pad is welded to the clamping surfacexe2x80x94resulting in the mirror 811 being stuck.
There is a need, therefore, for a MEMS device having stiction resistant landing pads and a method of operating a MEMS device configured in a stiction reduced mode.
The disadvantages associated with the prior art are overcome by a MEMs design having electrically isolated conductive landing pad structures that can be set to the same electrical potential as the landing surface. The design is enabled by providing a substrate having a sacrificial layer disposed between a base layer and a device layer. One or more vias are etched through the device layer and the sacrificial layer is etched forming depressions in the sacrificial layer at locations corresponding to vias in the device layer. The vias and depressions are filled with an electrically conductive landing pad material forming an isolated structure having landing pads that may be coupled to a voltage potential substantially equal to that of the landing surface.
The various embodiments of the present invention include methods of production and inventive devices having a device layer with at least one landing pad on an underside of the device layer attached to the device layer by a plug passing through an opening in the device layer. The device may be attached to the device layer by one or more compliant flexures, which allow the device to move in and out of a plane defined by the device layer.
The various embodiments are well suited to use with silicon on insulator substrates since the patterning of a sacrificial oxide layer may be performed either before or after bonding the device layer to the rest of the substrate, however other materials may be substituted by one skilled in the art.
Particular embodiments of this design may be applied to photonic switching applications using MEMs mirrors and other light path altering mediums.
The present invention includes one method of fabricating a mirror structure having landing pads made of a conductive material that are electrically insulated from the mirror and are kept at a potential equal to that of the landing surface. The landing pads may alternatively be located on the clamping surface, being electrically isolated from therefrom and kept at a potential equal to that of the mirror structure.
Alternative embodiments provide for a MEMS device having conductive landing pads on an underside of a flap, wherein the landing pads are electrically isolated from the flap and wherein the one or more landing pads are electrically coupled to a landing surface, e.g. a base or a sidewall. The device may optionally include conductive landing pads disposed on a top surface of the flap that are electrically isolated from the flap and electrically coupled to the sidewall. The device may also optionally include conductive landing pads on the base that are electrically isolated from the base and electrically coupled to the flap. Alternatively, the device may optionally include conductive landing pads disposed on the sidewall that are electrically isolated from the sidewall and electrically coupled to the flap.